The present invention relates to an apparatus and method for controlling the positioning of a first element or device with respect to a second element or device whose position is variable, and more particularly to such a positioning apparatus used for controlling the speed of a traveling web being driven along a path by means of first and second drive means located at spaced positions along the path.
It is often desirable, in the manufacture, treating or other processing of a web or strand of material to utilize a plurality of drive devices for moving or driving the web along a path between various treatment or processing stations. With such arrangements in which a web is driven at two points along its length by different drive apparatus, as can be appreciated it is highly desirable that both drive apparatus be operated at the same speed, as otherwise more web material will be driven past the upstream drive mechanism that is taken up by the downstream drive mechanism, resulting in possible spilling or tangling of the web, or the amount of web material delivered by the upstream drive mechanism to the downstream drive mechanism will be insufficient such that the web will be stretched and possibly damaged. While various prior art systems have employed the technique of slaving one drive mechanism to the other drive mechanism, either by electrical or mechanical means, these systems have not proved completely satisfactory, and the search has continued for a better, more reliable speed control apparatus.
In some systems, because of the nature of the web material being driven along the path, a catenary or loop is provided between the two drive mechanisms so that the web, in essence sags therebetween. The upstream drive mechanism delivers the web into the loop and the outlet or downstream drive mechanism takes up the web material as it exits from the loop or sag portion. This type of arrangement is useful, especially with fragile web materials which can not be subjected to tension since not only is a margin of time provided for correcting or equalizing the speeds of the two drive mechanisms, but additionally, it is possible to utilize the loop as a means for detecting variations in and controlling the speeds of the drive apparatus. More particularly, if the exiting drive mechanism is traveling at a greater speed than the input or inlet drive mechanism, then the amount of sag between the two drive mechanisms will slowly decrease. By detecting this decrease in sag height, it is then possible to correct the speed of the input drive mechanism. Similarly, if the input drive mechanism is traveling at a greater speed than the output drive mechanism, then the height of the sag or catenary will increase slowly. By detecting this increase in height, the speed of the output drive mechanism may be increased to that of the input drive mechanism, or alternatively, the speed of the input drive mechanism slowed to match the speed of the output drive mechanism. Thus, the position of the path of travel of the web is used to determine and control the speeds of the input and output drive mechanisms.
Various types of detection systems have been provided in the prior art for these types of systems, some of which operate on a contact principle, i.e., those in which the web is trained about an idler roller at the low point of the loop and the position of the idler roller detected (see for example U.S. Pat. No. 2,108,410). However, it should be realized that with many types of web materials, in particular webs of knitted material, it is not possible to use such an arrangement since the contact by the movable idler roller could result in stretching, distortion, or other damage to the web.
Other prior art arrangements or systems for monitoring the controlling the sag depth of a catenary traveling web positioned over a pair of spaced drive mechanisms are of the non-contact type. For instance, in U.S. Pat. No. 2,379,132, there is disclosed a photoelectric system for producing a control voltage responsive to movement of looped material extending between two work devices. The system includes a light source and optics for projecting a plurality of horizontally disposed beams of light at photoelectric devices, the beams being positioned so as to be intercepted by the lowest point of the loop as the sag depth of the loop changes. A control voltage which varies inversely with the depth of the sag of the loop is generated to control the speed of the take-up device to alter the sag depth of the loop.
However, it should also be noted in this regard that with such photoelectric device systems, the light source and photoelectric devices are spaced on opposite sides of the path of travel of the web so that the beam of light is directed across the path of travel. Thus, such systems rely on the web to provide a contrast in order to block the light as the web moves therepast. As can be appreciated, problems may be encountered with such contrast detection systems with respect to web materials which are transparent or highly porous since the light beams would in essence pass through the web material and not provide an indication of blockage by the web. Still further, such systems require a very precise alignment of the light sources and the detectors. Additionally, it will further be appreciated that with such an arrangement, the sensors, i.e., the photoelectric devices and light sources, are externally fixed with respect to the traveling web and thus can only provide for stepwise changes or alterations in the speed of the various drive mechanisms.
One system which does provide a type of speed control utilizing movable sensors is shown in U.S. Pat. No. 2,108,410 entitled "Speed Control Mechanism" to W. R. Perry. In one of the embodiments of this system, a pair of light sources and corresponding photodetectors are mounted inside a box-like structure in vertically spaced relationship, the light sources being mounted on one side of the box and the detectors mounted on the opposite side of the box. A strand being wound up travels about a grooved roller at the low point of the sag, which roller is adapted to move vertically along a path between the respective pairs of light sources and photodetectors. The box is also mounted for movement and controlled so that the grooved roller is confined at an elevation between the upper light source and its photodetector in the box and the lower light source and its photodetector box. Thus, if the strand is wound up at a speed greater than that at which it is fed, the grooved roller will rise, thereby causing an interruption in the circuit of the upper photodetector. This in turn causes an appropriate control mechanism to slow the speed of the take-up mechanism and at the same time to raise the box-like structure to clear the obstruction of the upper photodetector. Similarly, if the strand is fed at a speed greater than that at which it is being taken up, the groove roller will move downwardly, thereby intercepting the light beam of the lower light source. This in turn causes the take-up mechanism to increase the speed, and additionally results in the box being lowered to again position the two light beams above and below the grooved roller.
While such a mechanism does provide for movable sensing devices, it will be appreciated that the structure thereof is quite crude, and additionally, the arrangement still operates with detectors arranged on opposite sides of the strand. While such an arrangement may be useful in connection with strands of material which have a very small web width, it will be appreciated that this type of arrangement would encounter problems with very wide webs which simply hang freely between the input and output drive mechanisms. For instance, with a completely free hanging loop, the web may swing or move slightly as a result of surrounding air currents. Additionally, such a system necessarily depends upon a contrast being presented by the web as it passes through the beam of light, i.e., such an arrangement may not be satisfactory for a web of a construction so as to be transparent to the light radiation.
In U.S. Pat. No. 4,195,791, there is disclosed a catenary controller for controlling the sag depth of optical fiber strands which is being continuously moved between a feed and take-up apparatus. In this patent, the apparatus includes a light source for illuminating the strand in the vicinity of the lowest point of the catenary loop, and a stationarily fixed TV camera for raster scanning the catenary vertically to provide an output signal which is then utilized for controlling the speed of the take-up apparatus to maintain a predetermined sag depth. The TV camera is fixedly positioned so as to be laterally spaced from the strand and optically aligned with the lower portion of the catenary, whereas the light source is arranged to be substantially normal to both the tangent of the catenary and the optical axis of the camera.
However, as noted above, with many types of traveling web apparatus, it is desirable to operate the apparatus to provide loops capable of varying heights in order to accommodate rapid processing changes in speed of the web. The apparatus disclosed in this U.S. Pat. No. 4,195,791 is not adapted for use with varying sag heights. Additionally, the use of an illuminating source and TV camera is limited to only specific types of web materials which can be illuminated.
Consequently, there exists a need for an improved positioning apparatus and method which is particularly useful in controlling the speed of a web traveling along a path and driven by separate drive means.